(1) Field of the Invention
This invention generally relates to bovine genetics and, more particularly, to a means for determining the presence of a genetic marker in bovine genetic material indicative of an inheritable trait of increased milk production.
(2) Description of the Related Art
A primary goal of the dairy industry has been to identify an efficient and economical way to increase milk production without increasing the size of the dairy herd. The traditional approach features breeding programs designed for the purpose of breeding and selecting dairy cows capable of superior milk production. While such programs have proved to be successful in improving milk production, they are disadvantageous because of the significant costs involved before the success of the program can be determined. For example, a traditional breeding program requires the breeding of many cows with a particular bull and subsequent analysis of the milk production of the female progeny of these cows to determine whether the bull is of superior genetic value. Of course, the female progeny must be raised, become pregnant, allowed to give birth and milked for a minimum length of time before its milk production capabilities can be analyzed. Thus, a breeding program relying on traditional techniques and selection criteria typically requires the investment of 4 or more years in a group of cattle before significant analysis of the program can be undertaken. It would, therefore, be advantageous if additional methods or criteria were available to determine whether a bull, heifer or cow should be included in a breeding program designed for superior milk production.
A technique utilized in genetic analysis known as restriction fragment length polymorphism analysis has been shown to be useful in identifying differences or polymorphisms in genetic material in a population. A polymorphism is an allelic variation in the genetic code of a particular gene sequence. That is, at least two different forms or variants of a gene sequence exist among subjects in a single population. It has been shown that some polymorphisms in a gene sequence can be identified by its association with recognizable differences in restriction fragment lengths when the gene sequence is cut by a particular restriction endonuclease. Thus, by analyzing numerous subjects within a population to determine the form of the gene existing in individual subjects, discrete groups within a population can be identified. These individual groups can then be compared to determine whether a variant of the gene is associated with a particular phenotype or trait.
Recently, researchers utilizing this type of analysis have identified a genetic marker indicative of superior milk production in bovine. The marker involves the presence of a polymorphism adjacent to the bovine prolactin gene sequence and a means for determining the presence of such polymorphism in bovine genetic material is described in U.S. Pat. No. 5,041,371. While this marker is useful in providing information regarding the genetic potential of a bovine for the purpose of breeding and selecting for superior milk production, it is acknowledged that it is just one piece of information to be considered in an overall breeding program. It would, therefore, be advantageous if other genetic markers in bovine genetic material could be identified that provide additional indication of the genetic potential of a bovine being considered for inclusion in a breeding program or in a milking herd. It would be desirable if, ultimately, a plurality of markers indicative of a desired trait in dairy cattle, such as superior milk production, could be identified and developed. This would provide the basis for a breeding and selection program requiring only the genetic analysis of a bovine to determine its genetic potential. Bovine that exhibited the desired genetic potential could then be selected for use in a breeding program based upon such genetic analysis.
Four variants of the bovine somatotropin gene are known to exist in cattle. (Wood et al. (1989) Purification and characterization of pituitary bovine somatotropin. J. Biol. Chem. 264:14741-14747) These variants arise from the combination of two possible N terminal amino acids (alanine or phenylalanine); See Lingappa et al. (1977) Nascent prehormones are intermediates in the biosynthesis of authentic bovine pituitary growth hormone and prolactin, Proc. Natl. Acad. Sci. 74:2432-2436) with two possible amino acids at position 126 of somatotropin (leucine or valine); See Seavey et al. (1971) Bovine growth hormone: evidence for two allelic forms. Bioch. Biophys. Res. Comm. 43:189-195). As used herein, the numbering of the somatotropin protein is with respect to the somatotropin having phenylalanine as its first amino acid. Variation at the N terminus of somatotropin arises from the variability in the cleavage of the signal peptide. This results in somatotropin molecules that are initiated by either alanine or phenylalanine. There is no known genetic element responsible for the variability in this cleavage event. On the other hand, allelic variation exists for the somatotropin molecule at amino acid position 126. This variation is caused by a single nucleotide change in the codon for amino acid 126 of the somatotropin gene.
Applicants are unaware of any reports associating the allelic variation of the bovine somatotropin gene at amino acid position 126 with the genetic potential for increased milk production. It is known that recombinant forms of the somatotropin protein can be injected into female cattle to increase milk production. It is also known that when Holstein dairy cattle are injected with recombinant forms of somatotropin to increase milk production, a greater response occurs in cows injected with the valine.sub.126 variant as compared to the leucine.sub.126 variant (Eppard et al. (1992) Comparison of the galactopoietic response to pituitary-derived and recombinant-derived variants of bovine growth hormone. J. Endocrinol. 132:47-56). It should be understood that the polymorphism in the somatotropin gene at amino acid position 126 provides a marker that can be correlated to the trait of superior milk production, but does not necessarily identify the polymorphism as the cause of such trait. The actual cause of the increased milk production may be due to some other closely linked (i.e. in close proximity) genetic factor or gene in the bovine's genome, and not to the existence of the polymorphism.